Suppression of Spinel Formation to Induce Reversible Thermal

Similar considerations do not hold in the case of Ga- and In-containing LDHs, .... June 2005-LLB JRC).22 In all the refinements, the Pearson VII funct...
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J. Phys. Chem. B 2007, 111, 3384-3390

Suppression of Spinel Formation to Induce Reversible Thermal Behavior in the Layered Double Hydroxides (LDHs) of Co with Al, Fe, Ga, and In A.V. Radha,† Grace S. Thomas,† P. Vishnu Kamath,*,† and C. Shivakumara‡ Department of Chemistry, Central College, Bangalore UniVersity, Bangalore 560 001, India, and Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India ReceiVed: NoVember 15, 2006; In Final Form: January 24, 2007

The layered double hydroxides (LDHs) of Co with trivalent cations decompose irreversibly to yield oxides with the spinel structure. Spinel formation is aided by the oxidation of Co(II) to Co(III) in the ambient atmosphere. When the decomposition is carried out under N2, the oxidation of Co(II) is suppressed, and the resulting oxide has the rock salt structure. Thus, the Co-Al-CO32-/Cl- LDHs yield oxides of the type Co1-xAl2x/3 0x/3O, which are highly metastable, given the large defect concentration. This defect oxide rapidly reverts back to the original hydroxide on soaking in a Na2CO3 solution. Interlayer NO3- anions, on the other hand, decompose generating a highly oxidizing atmosphere, whereby the Co-Al-NO3- LDH decomposes to form the spinel phase even in a N2 atmosphere. The oxide with the defect rock salt structure formed by the thermal decomposition of the Co-Fe-CO32- LDH under N2, on soaking in a Na2CO3 solution, follows a different kinetic pathway and undergoes a solution transformation into the inverse spinel Co(Co,Fe)2O4. Fe3+ has a low octahedral crystal field stabilization energy and therefore prefers the tetrahedral coordination offered by the structure of the inverse spinel rather than the octahedral coordination of the parent LDH. Similar considerations do not hold in the case of Ga- and In-containing LDHs, given the considerable barriers to the diffusion of M3+ (MdGa, In) from octahedral to tetrahedral sites owing to their large size. Consequently, the In-containing oxide residue reverts back to the parent hydroxide, whereas this reconstruction is partial in the case of the Ga-containing oxide. These studies show that the reversible thermal behavior offers a competing kinetic pathway to spinel formation. Suppression of the latter induces the reversible behavior in an LDH that otherwise decomposes irreversibly to the spinel.

Introduction Metal hydroxides have been traditionally used as precursors for the low-temperature synthesis of metal oxides.1 Among the metal hydroxides, the layered double hydroxides (LDHs) having the formula [M(II)1-xM′(III)x(OH)2](An-)x/n‚yH2O (MdMg, Co, Ni; M′ ) Al, Fe; An- ) CO32-, NO3-, Cl-) have been widely studied, as precursors to oxide catalysts.2,3 A typical LDH with x ) 0.25 and An- ) CO32- has the formula [M(II)6M′(III)2(OH)16]CO3‚4H2O.4 We refer to this as M(II) -M′(III) LDH. Most LDHs decompose completely below 500 °C to yield an oxide residue. Thus, the Ni-Al LDH for instance decomposes in two steps as below:5

[Ni6Al2(OH)16]CO3‚4H2O f [Ni6Al2(OH)16]CO3 + 4H2O v (T ) 250 °C) (1) [Ni6Al2(OH)16]CO3 f 5NiO + NiAl2O4 + 8H2O v + CO2 v (T ) 450 °C) (2) The oxide residue comprises a mixture of rocksalt NiO and spinel NiAl2O4. Spinel formation is, however, not facile among the Mg-containing LDHs. The structure of the LDHs is based on that of mineral brucite, Mg(OH)2. Brucite comprises a stacking of charge neutral layers of Mg(OH)2 formed by hexagonal closely packed arrays of * Corresponding author. E-mail: [email protected]. † Bangalore University. ‡ Indian Institute of Science.

hydroxyl ions with Mg2+ ions occupying alternative layers of octahedral sites.6 In LDHs, these layers acquire positive charge with composition [Mg1-xM′(III)x(OH)2]x+ due to the isomorphous substitution of a fraction, x (0.2 e x e 0.33), of the Mg2+ ions by trivalent ions M′(III) (M′ ) Al, Cr, Fe, Ga, In). Anions, An- (A ) CO32-, NO3-,Cl-), are incorporated in the interlayer region for charge neutrality, resulting in the formation of LDHs with composition [Mg1-xM′(III)x(OH)2](An-)x/n‚mH2O. Many divalent ions such as Ca2+, Co2+, Ni2+, Cu2+, and Zn2+ can take the place of Mg2+ leading to a large family of compounds. Thus, all the cations in LDHs occupy octahedral sites. There is evidence to suggest that the dehydration and decomposition of the LDHs is topotactic in nature. Professor Figlarz and coworkers demonstrated by direct experimental evidence the topotactic relationship [001]Hex | [111]rocksalt.7 In view of this, oxides of rocksalt structure are the preferred products of LDH decomposition, as the octahedral cation coordination is conserved in the product having the rocksalt structure. There is therefore no clear understanding of the mechanism of spinel formation. The case of Mg-Al LDH is unique as MgAl2O4 is a normal spinel and the migration of Mg2+ from octahedral to tetrahedral coordination is associated with a positive free energy change. Thereby, the Mg-Al LDH decomposition takes place as

Mg6Al2(OH)16CO3 f Mg6Al2 0O9 + 8H2O v + CO2 v (T ) 450 °C) (3)

10.1021/jp067562a CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007

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TABLE 1: Precipitation Conditions Used for the Synthesis of Different LDH Samples LDH system

pH

T (°C)

anion source

post precipitation treatment

Co-Al-CO3 Co-Al-NO3 Co-Al-Cl Co-Fe-CO3 Co-Ga-CO3 Co-In-CO3

8 8 8 10 8.5 8.5

23 60 60 25 26 27

Na2CO3 NaNO3 NaCl Na2CO3 Na2CO3 Na2CO3

80 °C, 18 h 60 °C, 18 h 60 °C, 18 h 80 °C, 18 h 60 °C, 24 h 60 °C, 24 h

The oxide Mg6Al20O9 has a rocksalt structure and is the immediate product of the topotactic decomposition of the LDH following the Figlarz mechanism.7 This defect oxide on further heating transforms to the spinel:8

Mg6Al20O9 f 5MgO + MgAl2O4 (T > 1100 °C) (4) The defect rocksalt is highly unstable and rapidly reconstructs to the LDH on soaking in water,8 or on standing in air,9-10 by absorbing water vapor and CO2 from the ambient. This reversible decomposition of the Mg-Al LDH has been studied extensively using variable temperature PXRD,11-12 solidstate NMR,13-14 TGA,15 EGA,16 and isothermal heating studies.11 The reversible behavior has been observed among the Mg-Fe,17 Mg-Ga (In),18 Zn-Al,19 and Zn-Ga20 LDHs. The reversible behavior can be suppressed on sintering the LDH at temperatures above those required for spinel formation in each system.8,19 In this paper we give direct experimental evidence to show that spinel formation and reversible thermal behavior of the LDHs are competing reactions. LDHs such as those of Co, which exhibit facile spinel formation are known to decompose irreversibly.21 We show that by suppressing spinel formation in the Co-Al system, reversible thermal behavior could be induced. The role of the anions and trivalent cations in spinel formation is also investigated. Experimental Methods All LDHs were prepared by the method of coprecipitation at constant pH. In a typical preparation, 50 mL of the mixed metal (Co2+ + M′3+) nitrate solution (M′ ) Al, Fe, Ga, In) in 3:1 ratio was added to a 100 mL solution of NanA (A ) CO32-, Cl-, NO3-) containing three (ten for Cl- and NO3- anions) times the stoichiometric requirement of the desired anion. 1 N NaOH was simultaneously dispensed using a Metrohm Model 718 STAT titrino operating in the pH stat mode to maintain constant pH (7-8). All precipitations were carried out at constant temperature. N2 gas was bubbled through the solution continuously for the preparation of Cl- and NO3- containing LDHs to avoid carbonate contamination. The slurry so obtained was aged at 80 °C for 18 h and the solids were recovered by centrifugation, washed and dried at 80 °C. For noncarbonate LDHs, the precipitate was washed with warm decarbonated water several times and finally with acetone before being dried

Figure 1. PXRD patterns of (a) Co-Al-CO32- LDH, (b) Co-AlCO32- LDH decomposed in air at 400 °C and soaked in Na2CO3, and (c) Co-Al-CO32- LDH decomposed in N2 at 400 °C and soaked in Na2CO3. The feature marked by the asterisk corresponds to some residual spinel phase.

at 80 °C. Table 1 gives the precipitation conditions used for the synthesis of different LDH samples. Thermogravimetric studies were carried out using a MettlerToledo 851e TG/ SDTA system driven by Stare 7.1 software. Around 250 mg of the sample was taken in an alumina crucible and heated in the TG balance at the rate of 10 °C min-1 with 50 mL min-1 gas flow (ultrahigh-purity N2/air) up to 400 °C, followed by isothermal heating for 1 h at 400 °C. A part of this residue was subjected to reconstruction studies by soaking a known mass of the residue in 10 mL sodium carbonate solution (0.19 M) for 4 days at 60 °C. Then the residue was filtered using a sintered glass crucible, washed, dried and weighed. The results of mass gain studies are given in Table 2. All samples were characterized by Powder X-ray Diffraction (PXRD) studies using X’pert Pro Philips diffractometer (source Cu KR, λ ) 1.54 Å) with graphite monochromator. The samples were further characterized by infrared spectroscopy (Nicolet Model Impact 400D FTIR spectrometer, KBr pellets, resolution 4 cm-1) to verify the presence of intercalated anions. The PXRD patterns of the oxide residues obtained by thermal decomposition of the LDHs were subjected to Rietveld refinement studies using the FULLPROF.2k code (Version 3.3 June 2005-LLB JRC).22 In all the refinements, the Pearson VII function was used to fit the experimental profiles and a twelve co-efficient polynomial function to fit the background. The structures of MgO (ICSD CC ) 95468) and ZnAl2O4 (ICSD CC ) 94155) are used as model structures for rock salt and spinel phases. Results and Discussion Decomposition of Co-Al-CO32- LDH. In Figures 1a and 2a are shown the PXRD pattern and the IR spectrum of the

TABLE 2: Results of the TGA Study of the LDHs expected mass loss (%) LDH system

mass loss (%)in TG

CoO + M′2O3 formation

Co3O4 + M′2O3 formation

mass of residue on reconstruction (%)

Co-Al-CO3 (air) Co-Al-CO3 (N2) Co-Al-NO3 (N2) Co-Al-Cl (N2) Co-Fe-CO3 (N2) Co-Ga-CO3 (N2) Co-In-CO3 (N2)

29.1 32.4 32.0 27.7 29.8 29.6 24.7

32.0 32.0 37.0 32.9 29.9 29.0 26.3

28.1 28.1 33.3 29.0 26.2 25.4 23.1

79.5 93.7 70.1 94.0 99.3 >100 >100

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Figure 2. IR spectra of (a) Co-Al-CO32- LDH, (b) Co-Al-CO32LDH decomposed in air at 400 °C and then (c) soaked in Na2CO3, and (d) Co-Al-CO32- LDH decomposed in N2 at 400 °C and then (e) soaked in Na2CO3.

Co-Al-CO32- LDH. The PXRD pattern of the LDH can be indexed to a hexagonal cell with a ) 3.08 Å and c ) 22.98 Å. Further, the low-angle reflection at 7.7 Å (11.55 °2θ) in the PXRD pattern and an absorption at 1362 cm-1 in the IR spectrum (Figure 2a) confirm the presence of the intercalated carbonate ion. As shown by the TGA data (Figure 3a), this LDH decomposes completely well below 400 °C with a mass loss of 29.2% in air. Figure 4a shows the PXRD pattern of the sample obtained after decomposition in air at 400 °C. The absence of the basal reflections in the PXRD pattern and the disappearance of the peaks due to carbonate ν3 (1362 cm-1) and Co-Al-OH vibrations (743 cm-1 and 599 cm-1) in the IR spectrum (Figure 2b) indicate the disintegration of the LDH structure. The Rietveld refinement of the PXRD pattern (Figure 4a) of this decomposed sample shows that the oxide residue corresponds to a spinel phase of composition Co3O4. A total of 22 parameters were refined to get a reasonably good fit. The value of RBragg at 12% was high due to poor crystallinity of the residue. Further evidence for the spinel formation can be found in the IR spectrum, which shows a typical well-resolved doublet at 570 and 670 cm-1 (Figure 2b). The 670 cm-1 band corresponds to AOB3 and the 570 cm-1 band to BOB2 vibrations that are observed in a normal spinel compound. This oxide residue was soaked in a sodium carbonate solution. In order to facilitate the reconstruction process, vigorous reaction conditions were maintained (t ) 4 days, T ) 60 °C). The residue gains 8.6% mass. The PXRD pattern of the sample (Figure 1b) does not show any change from that shown in Figure 4a, indicating the conservation of the spinel structure. The infrared spectrum also remains unchanged from that recorded before soaking the residue (see Figure 2c). This control experiment clearly demonstrates the irreversible nature of thermal decomposition of the Co-Al-CO32- LDH in air in agreement with earlier results reported in the literature.21 Direct spinel formation among the Co containing LDHs is a result of oxidative decomposition:

Co6Al2(OH)16CO3 + 1/2 O2 f 5/3 Co3O4 + CoAl2O4 + (T ) 240 °C) (5) 8 H2O + CO2

Figure 3. TG-DTG curves of the Co-Al-CO32- LDH (a) in air (b) in N2 and (c) Co-Al-NO3- LDH in N2.

Oxidative decomposition suppresses the formation of the divalent monoxide seen in the Ni-based LDHs. Co3O4 and CoAl2O4 being thermodynamically more stable than the LDHs, the spinels do not reconstruct the LDH structure. It was our surmise that reversible thermal behavior could be induced in the Co-Al LDH system by suppressing spinel formation, by carrying out the thermal decomposition of the LDH in N2 atmosphere. Figure 3b shows the TGA of Co-Al-CO32- LDH under nitrogen flow. The TG shows a well-defined two-step mass loss. The total mass loss observed (32.4%) in the TG corresponds to that expected for CoO + Al2O3 formation (32.0%). The PXRD pattern of the sample obtained after thermal decomposition of Co-Al-CO32- LDH at 400 °C under nitrogen atmosphere is given in Figure 4b. The absence of low-angle reflections in the PXRD of the decomposed sample indicates the collapse of the LDH structure. The PXRD pattern at Figure 4b, is different in character from its counterpart given in Figure 4a and is closer to that expected of CoO. The profile at Figure 4b was analyzed by the Rietveld refinement procedure using the structure of CoO as the model (not shown). The resulting difference profile showed a residual intensity in the

Suppression of Spinel Formation

Figure 4. Rietveld refinements of the structures of the oxide residues obtained from the Co-Al-CO32- LDH at 400 °C in (a) air and (b) N2.

30-40° range of 2θ. The RBragg at 30% was also unsatisfactory. The Rietveld refinement was repeated including the spinel as a second phase and the results are given in Figure 4b. The fit now corresponds to a mixture of the defect rock salt phase with composition Co0.7Al0.200.1O (RBragg ) 13%) along with a residual Co3O4 spinel phase (RBragg ) 21%) (Figure 4b). The presence of residual spinel is due to the oxidation Co2+ during dehydroxylation of the LDH. Further, absence of the wellresolved doublet (570 and 670 cm-1) in the IR spectrum of the decomposed sample (Figure 2d), confirms the absence of spinel as a major phase. Figure 1c shows the PXRD pattern of the sample soaked in sodium carbonate solution. The emergence of the low angle basal reflection at 7.7 Å in the PXRD pattern and a characteristic absorption band at 1360 cm-1 in the IR spectrum (Figure 2e) indicates the reconstruction of the CoAl-CO32- LDH structure. The reconstructed residue regains 93.7% of the original mass after soaking in sodium carbonate. These results clearly demonstrate the reversible thermal behavior of Co-Al-CO32- LDH in nitrogen atmosphere due to suppression of the Co3O4 phase formation. Co3O4 suppression could be effectively achieved by (i) starving the LDH of oxygen during decomposition and (ii) the reducing atmosphere of CO2 generated during the decomposition of the Co-Al-CO32- LDH. Decomposition of Co-Al-NO3-/Cl- LDHs in N2 Atmosphere.The second factor above could be nullified in the CoAl-NO3- system, as the nitrate ion decomposes generating a highly oxidizing atmosphere. The Co-Al-NO3- LDH also decomposes completely at 400 °C in N2 (see Figure 3c). In

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Figure 5. PXRD patterns of (a) Co-Al-NO3- LDH, (b) Co-AlNO3- LDH decomposed in N2 at 400 °C and soaked in Na2CO3, and (c) Rietveld fit of Co-Al-NO3- LDH after decomposition in N2 at 400 °C.

Figure 5 are shown the PXRD patterns of the as-prepared LDH and the products of subsequent decomposition/ reconstruction reactions carried out under N2. The PXRD pattern of the CoAl-NO3- LDH (Figure 5a) shows an interlayer distance of 8.2 Å (a ) 3.09 Å; c ) 24.6 Å). The IR spectrum of the Co-AlNO3- LDH (Figure S1 of Supporting Information) shows a sharp absorption at 1387 cm-1 due to intercalated nitrate ions. The Rietveld refinement of the PXRD pattern (Figure 5c) of the oxide residue obtained from this LDH corresponds to a single-phase spinel with composition Co3O4. On soaking in Na2CO3 solution, this phase does not reconstruct back to the original LDH (see Figure 5b) (also see Figure S1 of Supporting Information for IR data). In contrast the Co-Al-Cl- shows reversible thermal behavior on decomposition under N2 (see Figures S2 and S3 of Supporting Information) These results clearly show that the intercalated anions play an active role in the thermal decomposition of Co-Al LDH. The intercalated Cl- anions induce reversible thermal behavior, whereas the intercalated nitrate ions hinder the reversible thermal behavior in the Co-Al LDH system by generating a highly oxidizing atmosphere during decomposition. The thermal decomposition of the LDHs of Co with Fe, Ga, and In in air parallels the behavior of the Co-Al LDH. Will the behavior of these LDHs in N2 follow the same trend? We investigate. Decomposition of Co-Fe-CO32- LDH in N2 Atmosphere. Figure 6a shows the PXRD pattern of the precursor Co-FeCO32- LDH, and it matches well with the published structure

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Figure 7. PXRD patterns of the (a) Co-In-CO32- LDH, (b) CoIn-CO32- LDH decomposed in N2 at 400 °C and soaked in Na2CO3, and (c) Rietveld fit of Co-In-CO32- LDH decomposed in N2 at 400 °C (*, due to In2O3).

Figure 6. PXRD pattern of the (a) Co-Fe-CO32- LDH compared with the Rietveld fits of its oxide residue at 400 °C in N2 (b) before and (c) after soaking in Na2CO3.

of this LDH (a ) 3.12 Å and c ) 22.84 Å). This LDH was decomposed at 400 °C under N2 flow (see Figure S4 of Supporting Information for TG data). The PXRD pattern of the decomposed sample is given in Figure 6b. The Rietveld refinement of the PXRD pattern of this residue corresponds to a mixture of defect rock salt Co0.6Fe0.200.2O and spinel Co3O4 phases. The PXRD pattern of the oxide residue soaked in sodium carbonate is shown in Figure 6c. In contrast with its Al counterpart, the soaked sample does not show any reflection related to LDH phase. In fact, the results of Rietveld refinement of the structure of this sample indicates the growth of spinel phase at the expense of the defect rock salt phase. The profile could be fit to a mixture of Co(Fe,Co)O4 and Co0.900.1O phases, the former being predominant. In presence of Fe3+, the defect rock salt oxide transforms to the inverse spinel and does not reconstruct the original LDH on soaking in sodium carbonate solution (see Figure S5 of Supporting Information for IR results). Decomposition of Co-M3+-CO32- LDH (MdIn, Ga) in N2 Atmosphere. In our recent paper on Ga- and In-containing LDHs, we have discussed the origin of irreversible thermal decomposition of the Co-containing LDHs in air.18 It is interesting to look at the decomposition behavior of these LDHs

in N2. Figures 7a and 8a give the PXRD patterns of the CoIn-CO32- and Co-Ga-CO32- LDHs. The patterns can be indexed on hexagonal cells [Co-In LDH: a ) 3.22 Å and c ) 22.98 Å; Co-Ga LDH: a ) 3.132 Å and c ) 23.34 Å]. The TG data show these LDHs decompose completely well before 400 °C in N2 (see Figure S4 of Supporting Information). The Rietveld refinements of the decomposed oxides of Co-In and Co-Ga LDHs at 400 °C are given in Figures 7c and 8c, respectively (see Figures S6 and S7 of Supporting Information for IR results). Here again the patterns can be fit to a biphasic mixture of oxides having rock salt and spinel structures. For Co-In it corresponds to a mixture of Co0.59In0.200.21O and spinel Co3O4 phases, and for Co-Ga it is a mixture of Co0.77Ga0.200.03O and spinel Co3O4 phases. Interestingly, these oxide residues when soaked in sodium carbonate solution reconstruct back to the LDH structure completely in Co-In system (Figure 7b) but partially in case of Co-Ga system (Figure 8b). The compositions of the defect rock salt phases provided in the preceding paragraphs are simply the results of refinement of the occupancy factors in the Rietveld procedure. Given the poor crystallinity of the oxide residues, these compositions are indicative rather being definitive. Suffice to say that when the PXRD profile of the oxide residue is due to a single phase spinel, the decomposition is irreversible. When the oxide residue shows reversible behavior, two phases, one of which is a rock salt, have to be invoked to fit the PXRD profile satisfactorily.

Suppression of Spinel Formation

Figure 8. PXRD patterns of the (a) Co-Ga-CO32- LDH, (b) CoGa-CO32- LDH decomposed in N2 at 400 °C and soaked in Na2CO3, and (c) Rietveld fit of Co-Ga-CO32- LDH decomposed in N2 at 400 °C (*, due to Co3O4).

Among the LDHs of Co, formation of the spinel is favored because (a) The large octahedral CFSE of low spin d6 configuration of Co3+ ion facilitates the easy oxidation of Co2+ to Co3+, which is otherwise an endothermic reaction involving high ionization energy. (b) The Co2+ ions readily occupy tetrahedral sites in the spinel structure as there is only a small difference in LFSEs between tetrahedral and octahedral coordination for Co2+ ion. (c) Rocksalt CoO is stable at temperatures above 900 °C. These factors favor the formation of a spinel over CoO in air, as it requires less activation energy. The spinel is thermodynamically stable and consequently, the Co-based LDHs decompose irreversibly in air. CoO is known to crystallize both in the rock salt as well as in the wurtzite structure. Wurtzite CoO is a metastable phase and transforms to the rocksalt phase at 320 °C in N2 atmosphere.25 The formation of a rock salt CoO phase can be realized in an inert atmosphere. This is exactly what happens when CoAl-CO32- and Co-Al-Cl- LDHs are thermally decomposed in N2 atmosphere. The formation of this metastable CoO phase is responsible for the reversible thermal behavior of these LDHs. But the Co-Al-NO3- LDH decomposes irreversibly under similar conditions. This behavior of Co-Al-NO3- LDH is ascribed to the self-redox decomposition of the intercalated nitrate ions. The following reactions are expected take place under such conditions:

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2NO3- f NO + NO2(g) + O2-

(6)

NO3- + 1e- f NO2(g) + O2-

(7)

Co2+ f Co3+ + 1e-

(8)

The nitrate ion decomposes at about 190 °C to form NO2, creating a highly oxidative atmosphere. This leads to the formation of Co3O4 even in an inert atmosphere. Co-Fe-CO32- LDH is a unique system. It does not show reversible behavior despite the formation of the defect rock salt phase. Given the near zero octahedral CFSE of Fe3+ ions, these prefer to diffuse into the tetrahedral sites offered by the inverse spinel from the octahedral sites of the defect rock salt phase. There are substantial free energy gains by this transformation as a consequence of which the transformation takes place in a facile manner in solution at ambient temperature. Solution transformation into the inverse spinel structure comprises an alternative kinetic pathway for the defect rock salt. This clearly indicates that the formation of the metastable defect rocksalt phase is not the only criterion to induce the reversible thermal behavior in LDHs. The final phase formation depends on the relative stabilities of possible phases as well as the kinetic path available for its formation. In3+ has a high tetrahedral CFSE which favors the inverse spinel structure. Despite this the oxide residue of the Co-In LDH completely reconstructs to the parent structure. This can be attributed to the large activation energy associated with the movement of the larger In3+ ions from octahedral sites of the defect rock salt structure to tetrahedral sites in the spinel. The lack of a kinetic path with appropriate activation energy hinders the spinel formation and hence the oxide residue reverts back to the LDH structure. The Co-Ga LDH falls in between the Co-Fe and Co-In systems. Ga3+ also readily forms an inverse spinel with Co. The oxide residue of this system partially reconstructs the LDH structure. Conclusions Suppression of the spinel formation on thermal decomposition of Co based LDHs in nitrogen leads to the formation of a metastable oxide with rock salt structure. The formation of this phase does not necessarily lead to the reconstruction of the LDH structure. The final phase formation is determined not only by relative thermodynamic stabilities of the LDHs and the spinel phases but also on the kinetic pathways leading to the formation of these phases. Acknowledgment. The authors thank the Department of Science and Technology (DST), Government of India (GOI) for financial support. G.S.T. thanks the University Grants Commission (UGC), GOI for the award of a Teacher Fellowship under the FIP program. Supporting Information Available: Figures showing IR spectra, PXRD patterns, and TG-DTG curves. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Vidyasagar, K.; Gopalakrishnan, J.; Rao, C. N. R. J. Solid State Chem. 1985, 58, 29. (2) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (3) Vaccari, A. App. Clay Sci. 1999, 14, 161. (4) Carrado, K. A.; Kostapapas, A.; Suib, S. L. Solid State Ionics 1988, 26, 77.

3390 J. Phys. Chem. B, Vol. 111, No. 13, 2007 (5) Kannan, S.; Narayanan, A.; Swamy, C. S. J. Mater. Sci. 1996, 31, 2353. (6) Oswald, H. R.; Asper, R. In Preparation and Crystal Growth of Materials with Layered Structures; Lieth, R. M. A., Ed.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1977; Vol. 1, p 71. (7) Figlarz, M.; Gerand, B.; Delahaye-Vidal, A.; Dumont, B.; Harb, F.; Coucou, A.; Fievet, F. Solid State Ionics 1990, 43, 143. (8) Sato, T.; Kato, K.; Endo, T.; Shimada, M. React. Solids 1986, 2, 253. (9) Puttaswamy, N. S.; Kamath, P. V. J. Mater. Chem. 1997, 7, 1941. (10) Rocha, J.; del Arco, M.; Rives, V.; Ulibarri, M. A. J. Mater. Chem. 1999, 9, 2499. (11) Ennadi, A.; Legrouri, A.; De Roy, A.; Besse, J. P. J. Soid. State Chem. 2000, 152, 568. (12) Kannan, S.; Kishore, D.; Hadjiivanov, K.; Knozinger, H. Langmuir 2003, 19, 5742. (13) Rey, F.; Fornes, V.; Rojo, J. M. J. Chem. Soc., Faraday Trans. 1992, 88, 2233.

Radha et al. (14) Tichit, D.; Bennani, M. N.; Figueras, F.; Ruiz, J. R. Langmuir 1998, 14, 2086. (15) Constantino, V. R. L.; Pinnavaiah, T. J. Inorg. Chem. 1995, 34, 883. (16) Bera, P.; Rajamathi, M.; Hegde, M. S.; Kamath, P. V. Bull. Mater. Sci. 2000, 23, 141. (17) Hibino, T.; Tsunashima, A. J. Mater. Sci. Lett. 2000, 19, 1403. (18) Thomas, G. S.; Kamath, P. V. Mater. Res. Bull. 2005, 40, 671. (19) Kooli, F.; Depege, C.; Ennaqadi, A.; de Roy, A.; Besse, J. P. Clays Clay Miner. 1997, 45, 92. (20) Thomas, G. S.; Kamath, P. V. Solid State Sci. 2006, 8, 1181. (21) Perez-Ramirez, J.; Mul, G.; Kapteijn, F.; Moulijn, J. A. Mater. Res. Bull. 2001, 36, 1767. (22) http://www-llb.cea.fr/fullweb/powder.htm. (23) Figlarz, M. Mater. Sci. Forum 1994, 152-153, 55. (24) Figlarz, M. Prog. Solid State Chem. 1989, 19, 1. (25) Risbud, A. S.; Snedeker, L. P.; Elcombe, M. M.; Cheetham, A. K.; Seshadri, R. Chem. Mater. 2005, 17, 834.